I 4 6 APES, MONKEYS, AND LEMURS 



cousins of the Old World in having all their digits provided with well-developed 

 nails. 



We have left to the last the most important and perfectly constant distinction 

 between the monkeys of the Old and New World, since it is one which can only be 

 observed in the dried skulls. It will be remembered that in our description of the 

 characteristics of the Man-like Apes, it was stated (p. 19) that in these and all the 

 monkeys of the Old World, the total number of teeth was thirty-two. Of these, on 

 each side of both upper and lower jaws, two were incisors, one was a canine, two 

 were premolars, and three molars ; the series being expressed by the formula z'f , c\ y 

 p\, m\ : total, 32. 



If we now examine the skull of any American monkey (always excluding the 

 marmosets) and count the teeth, we shall find that their total number is thirty-six, 

 or four more than in the Old World monkeys. A closer examination will show 

 that the additional tooth on each side belongs to the premolar series the so-called 

 bicuspids of human dentistry. Thus, whereas all Old World monkeys have but two 

 bicuspids on each side of both the upper and lower jaw, the American monkeys 

 have three of these teeth ; and the number of teeth in the latter may accordingly 

 be expressed by the formula z'f , c\, p\, m\ : total, 36. 



If we care to carry our examination a little further, we shall not fail to notice 

 that the upper molar teeth of the American monkeys differ very decidedly in the 

 form of their crowns from those of the monkeys of the Old World, so that a single 

 detached specimen of one of these teeth is amply sufficient to decide to which of 

 the two groups its owner belonged. Thus, whereas in the Old World monkeys 

 (exclusive of the Man-like Apes) the crowns of these teeth are tall and narrow, with 

 the four tubercles arranged in pairs nearly at right angles to the long axis, and 

 each tubercle nearly conical, in the monkeys of the New World the crowns of 

 these teeth are much shorter and broader, with their pairs of tubercles arranged 

 obliquely to the long axis ; the outer tubercles being much flattened, and the inner 

 crescent-shaped. Those acquainted with the details of anatomy will also find 

 characteristics by which the skulls themselves of the Old and New World monkeys 

 can be mutually distinguished. 



Having now shown the leading characteristics by which the 

 American monkeys, as a whole, are distinguished from those of the 

 Old World, we may refer to a few other matters before proceeding to the description 

 of the various species. 



In the first place, none of the American monkeys make any approach in point 

 of size to the large Man-like Apes, or even the baboons of the Old World. Then, 

 again, the whole of them are essentially adapted for a purely forest life. Indeed, 

 in the great primeval forests of the Amazon, where the ground is either swampy 

 or entirely under water, the monkeys, together with several other animals, pass 

 the whole of their lives in the tree tops, traveling from tree to tree, and rarely, if 

 ever, descending to the ground. 



In this purely arboreal life it will be easily seen that the prehensile tail of 

 those species which possess such an organ must be a great assistance to their 

 owners in traveling from bough to bough, and thus from tree to tree. Considering, 



